A known electric power supply circuit for a car stereo apparatus employs such a system that the power supply circuit is adapted, when the power is on, to supply required voltages to respective component boxes of the car stereo apparatus in response to operation of an external switch. The known system, however, has a disadvantage that the electric power supply system is complicated and a large number of electric power connecting cords are required in the system. The system has another disadvantage that the system cannot be applied to a part of the circuits constituting the car stereo apparatus.